Joe Hin Tjio

Modern cytogenetics, the branch of genetics studying the relationship between heredity, variation and the structure of chromosomes, began in 1956 when the Java-born scientist Joe Hin Tjio, who has died aged 82, made the first correct count of chromosomes in human cells. The breakthrough, which rewrote the textbooks, showed that the majority of cells in the body normally contain 46 chromosomes, arranged in 23 pairs. For the previous half-century, the accepted figure had been 48.

Tjio was a brilliant experimentalist. His achievement came with the development of delicate new techniques for the intricate job of separating chromosomes from the nucleus of the cell. The feat helped transform cytogenetics - essential to establishing the link between abnormal chromosomes and certain diseases - into a field of major medical importance in 1959, with the discovery that an additional chromosome was present in patients with Down's syndrome.

Tjio had an unusual career. He was born to Chinese parents in what was then part of the Dutch East Indies. His father was a professional portrait photographer, and learning to develop photographs and make prints in the darkroom proved a valuable asset for his son when he came to photographing microscope images.

Educated in strict Dutch colonial schools, Tjio learnt French, German and English, as well as Dutch. After taking an agriculture degree at a college in Bogor, Indonesia, he became involved in potato breeding. His research aimed to develop hybrid plants resistant to a common disease.

In 1942, he was imprisoned for three years in a Japanese concentration camp. Once released, he got a passage on a Red Cross boat for displaced persons going to Holland, where the government provided him with a fellowship for study in Europe.

He restarted his work on plant breeding in the Netherlands, and began collaborating with groups in Copenhagen and Sweden. His summers were spent at the Institute of Genetics, in Lund, Sweden, where he began an association with Albert Levan, the head of the laboratory. Levan encouraged Tjio to extend his interests from plants and insect cytogenetics to the study of mammalian tissues.

Tjio soon established a reputation in plant genetics. In 1948, the Spanish government invited him to work on a plant improvement programme and, for the next 11 years, he directed cytogenetics research in Zaragoza. During his holidays, he continued to research at Lund with Levan, and, although they were jointly recognised for the discovery of the correct chromosome count, the circumstances were unfortunate.

Tjio recalled his "serendipitous discovery" in the early hours of December 22 1955, when he was seeking to extract the chromosomes from the nuclei of some cells. He was trying an idea for refining an established technique to separate the chromosomes on to glass slides. When he looked at the slides under a microscope, he found improvements that yielded startling results.

He could count quite clearly that, in human embryonic lung tissue, there were 46 chromosomes rather than 48. The acceptance of the previous error had many consequences; among other things, it had led to researchers abandoning projects that did not seem to conform.

In great excitement, Tjio showed his results to Swedish colleagues, because Levan was on leave. He recalled their unanimous verdict was that he should publish his findings immediately. This was the start of another harrowing chapter, which Tjio maintained was to rival his imprisonment in unhappiness.

In the rush to publish - with himself as first author on the paper - he broke a long-standing convention of European universities. The protocol gave the first name to the head of a laboratory, in order to credit the work of his laboratory and acknowledge his support and guidance.

Tjio refused Levan first au thorship on the grounds that Levan had contributed nothing but resources to the work. To press his claim, Tjio threatened to throw his work away, challenging Levan to replicate the results: "If you want to be the author, you do the work." Levan ultimately gave way, on the grounds that the work belonged to science, and the paper was published in the Scandinavian journal, Hereditas, on January 26 1956.

The groundbreaking implications of the research were instantly recognised in the world of human cytogenetics, and Tjio was soon fielding offers to speak and teach. At the first International Human Genetics Congress, in Copenhagen in 1956, Herman Muller, Nobel laureate in genetics and professor at Indiana University, asked him to consider emigrating to the United States. Tjio declined at first, because the US was in the grips of McCarthyism.

But Muller persisted; he sent Tjio clippings critical of McCarthy from the New York Times, and insisted that "we are not all McCarthyites". In 1957, Tjio relented and took a research post at the University of Colorado. Soon afterwards, he was invited to join the US National Institute of Arthritis and Metabolic Diseases Laboratory of Experimental Pathology at Bethesda, Maryland. At NIH (National Institutes for Health), he extended his chromosome work, studying leukaemia and mental retardation.

In February 1992, Tjio retired with the status of scientist emeritus, retaining his space and resources until his 78th birthday. He moved then only because the NIH wanted to rebuild the laboratory.

The scientific legacy he left to the NIH includes a remarkable series of photographs summarising his scientific career. They document, in plants, insects, mammals and man, chromosomes frozen at precisely that moment in division when they could be seen distinctly as separate, wondrous entities.

Tjio is survived by his Icelandic wife Inga and a son.

· Joe Hin Tjio, cytogeneticist, born November 2 1919; died 27 November 2001.